Photolithography is a process used in semiconductor device microfabrication to selectively remove parts of a thin film or of the bulk substrate. The process uses radiation to transfer a geometric pattern from a photomask to a radiation-sensitive chemical photoresist or another radiation sensitive layer on the thin film or substrate. Typical radiation sources include optical, ultraviolet, x-ray and electron beam sources. Radiation sources commonly used in immersion lithography emit peak wavelengths at 193 nm. However, other wavelengths are possible.
To produce patterns with extremely small pitches in a photoresist, a phase shifting mask (PSM) may be used in conjunction with a dipole illumination. PSMs cause the shifting of the phase of a radiation source so that the peaks of one wave of radiation emitted by the radiation source lines up with the valleys of an adjacent wave (dipole illumination), effectively canceling each other out and producing a dual-beam image (a “shadow” image) between the waves that is smaller than the two waves themselves. The dual-beam image may be used to fabricate patterns having pitches as low as one-half the theoretical minimum pitch of the radiation source. In the PSM fabrication technique, radiation source beams are transmitted through zero degrees and 180 degrees and, when passing through the PSM mask, result in cancellation of the zero degree order of the radiation.
The highest resolution (smallest pitch size) currently achievable is produced using dipole illumination. Dipole illumination, however, is generally avoided when printing two dimensional (“2D”) features, such as holes, because dipole illumination is asymmetric. That is, dipole illumination favors one direction. Thus, dipole illumination is most suitable for printing one dimensional (“1D”) features.
It is very difficult to produce patterns with 2D features having a pitch less than 100 nm with conventional dipole illumination photolithographic techniques. It would therefore be advantageous to have a photolithographic technique suitable for patterning 2D features with the resolution of dipole illumination.